Most ad networks will give you a 1099-MISC (independent contractor) form. You are responsible for reporting all of that income to the IRS and pay taxes for it. You can mark off business expenses such as hosting, advertising, web design, etc etc to reduce your gross income.
Thats a 3 sentence guide to income tax... you will obviously find a lot better information on sites like inc.com, forbes.com, entrepreneur.com, fastcompany.com, etc.
Hope that helps somewhat